I don't think there is a hard fast rule. Use to be you divided it into 3rds to reduce stress on te seam, or so I have been told. But I do it however I can with whatever amount of fabric that I have. My main consideration is not to waste fabrci by cutting it in such a way to have 3rds when if I used another method, I might have a bigger scrap left over. Did I make sense? Crosswise or lengthwise, halves or thirds, it is up to you. I saw some quilts at a quilt show that had a hodgepodge of fabric for the back and the owners were quite pleased with the result. The quilt police have been bannished so please yourself.